After only a somewhat recent introduction, optical fiber has had a meteoric rise as the predominant means of transmission media in voice and data communications. Optical fiber is manufactured by drawing the fiber from a preform which is made by any of several well known processes. Afterwards, or as part of a tandem process, the drawn fiber is coated, cured, measured and taken up, desirably in an automatic takeup apparatus, on a spool. Methods and apparatus for taking up optical fiber are disclosed and claimed in U.S. Pat. No. 4,798,346 which issued on Jan. 17, 1989 in the names of D. L. Myers and J. G. Wright. Typically, an optical fiber has a diameter on the order of 125 microns, for example, and is covered with a coating material which increases the outer diameter of the coated fiber to about 250 microns, for example.
The optical fibers thus produced may be assembled into units and provided with a sheathing system. Or a plurality of optical fibers may be assembled into a planar array and secured in place by a curable matrix material such as is disclosed in application Ser. No. 213,876 which was filed on June 30, 1988 in the names of K. W. Jackson, et al, now U.S. Pat. No. 4,900,126. This last described product is referred to as a bonded ribbon.
In both the process for coating the optical fiber after it is drawn, which may include the application of primary and secondary curable coating materials, and in the process for embedding an array of optical fibers in a cured matrix material, it becomes important to be able to determine the degree of cure of the curable coating or matrix material.
Suitable curing of the optical fiber coating and of the ribbon matrix material is very important. An improperly cured bonded ribbon matrix or optical fiber coating material will inhibit cabling. Also an undercured coating or matrix material generally will emit an undesirable odor. Further, improperly cured coating and matrix materials exhibit poor strippability, poor adhesion and poor reliability relative to a properly cured coating and matrix material. As should be evident, properly cured coating and matrix materials for drawn optical fibers and for optical fiber ribbons are important to the quality of the final product. Further, the modulus of the cured of material is a function of the degree of cure. Modulus is important to the mechanical and optical performance of the optical fiber and fiber ribbon.
Presently, there are several methods which are used to determine the degree of cure of optical fiber coating materials. In one method, which is referred to as in-situ modulus test, a cut is made through the coating or coating layers to the glass. A load is applied to the optical fiber and measurements are taken as to the time during which the load is applied to the fiber and the distance through which the optical fiber moves. From these measurements, the modulus of the primary coating can be determined.
In a pull-out test, a length of optical fiber is positioned in a well known Instron apparatus. The force which is necessary to pull the optical fiber from the primary coating material is measured. This measurement is proportional to the degree of cure. In a third method which is referred to as the cut-through method, a length of coated optical fiber is placed into a fixture which prevents rolling of the fiber. A knife probe is caused to engage the fiber and that temperature at which the knife edge probe penetrates the coating material is measured. The greater the measured temperature, the greater the degree of cure.
There are several problems with respect to the use of the foregoing methods. First, each requires the use of relatively expensive equipment. Secondly, each is a so-called off-line test and hence is most likely not performed on 100 percent of the product throughput. Thirdly, a high level of employee training is required and lastly, the tests are operator sensitive.
Needless to say, it is most desirable that in-line testing for the degree of cure be accomplished as the optical fiber or optical fiber ribbon is being moved along a path of travel to a takeup. Bonded ribbon production line speeds may be in the range of about 200 feet per minute. If an off-line test were used to determine the degree of cure and hence the modulus, and if that test were to consume about five minutes, then it can be seen that about 12000 feet of optical fiber, assuming a twelve fiber ribbon, may be wasted if the degree of cure is not sufficient.
The problem of determining the degree of cure of a curable coating material exists in industries other than that for optical fiber manufacture. For example, curable materials are used in the manufacture of floor tiles, furniture, medical syringes, compact discs, computer floppy discs, video and audio tapes and glass fiber composites for automobiles and other products.
What is needed and what is not provided by the prior art are methods of monitoring the degree of cure of curable coating materials. What is especially sought after are methods of determining the degree of cure of curable coating and matrix materials which are used to provide optical fiber transmission media. Desirably, the sought after products and methods and apparatus for making same should be capable of being performed as the optical fiber or as the optical fiber ribbon is being moved along a manufacturing line.